Description

The tsearch(), tfind(),
tdelete(), twalk(), and tdestroy() functions
are routines for manipulating binary search trees. They are generalized
from Knuth (6.2.2)
Algorithms T and
D. All comparisons are done with a user-supplied routine.
This routine is called with two arguments, the pointers to the elements
being compared. It returns an integer less than, equal to, or greater
than 0, according to whether the first argument is to be considered
less than, equal to or greater than the second argument. The comparison
function need not compare every byte, so arbitrary data may be contained
in the elements in addition to the values being compared.

The tsearch() function is used to build and
access the tree. The key argument is a pointer
to a datum to be accessed or stored. If there is a datum in the tree
equal to *key (the value pointed to by
key), a pointer to this found datum is returned. Otherwise,
*key is inserted, and a pointer to it returned. Only pointers
are copied, so the calling routine must store the data. The rootp
argument points to a variable that points to the root
of the tree. A null value for the variable pointed to by rootp
denotes an empty tree; in this case, the variable will
be set to point to the datum which will be at the root of the new
tree.

Like tsearch(), tfind() will
search for a datum in the tree, returning a pointer to it if found.
However, if it is not found, tfind() will return
a null pointer. The arguments for tfind() are the
same as for tsearch().

The tdelete() function deletes a node from
a binary search tree. The arguments are the same as for tsearch
(). The variable pointed to by rootp will
be changed if the deleted node was the root of the tree. tdelete
() returns a pointer to the parent of the deleted node, or
a null pointer if the node is not found.

The twalk() function traverses a binary search
tree. The root argument is the root of the
tree to be traversed. (Any node in a tree may be used as the root
for a walk below that node.) action is the
name of a routine to be invoked at each node. This routine is, in
turn, called with three arguments. The first argument is the address
of the node being visited. The second argument is a value from an
enumeration data type

typedef enum { preorder, postorder, endorder, leaf } VISIT;

(defined in <search.h>), depending on
whether this is the first, second or third time that the node has
been visited (during a depth-first, left-to-right traversal of the
tree), or whether the node is a leaf. The third argument is the level
of the node in the tree, with the root being level zero.

The pointers to the key and the root of the tree should be of
type pointer-to-element, and cast to type pointer-to-character. Similarly,
although declared as type pointer-to-character, the value returned
should be cast into type pointer-to-element.

The tdestroy() removes the whole tree pointed
to by root, freeing all resources allocated by the tsearch() function.
For the key datum in each tree node, the function free_action() is
called. The pointer to the key is passed as the argument to the function.
If no such work is necessary free_action can be
NULL.

Return Values

If the node is found, both tsearch() and
tfind() return a pointer to it. If not, tfind() returns
a null pointer, and tsearch() returns a pointer
to the inserted item.

A null pointer is returned by tsearch() if
there is not enough space available to create a new node.

A null pointer is returned by tsearch(),
tfind() and tdelete() if rootp is
a null pointer on entry.

The tdelete() function returns a pointer
to the parent of the deleted node, or a null pointer if the node is
not found.

The twalk() and tdestroy() functions
return no value.

Errors

No errors are defined.

Usage

The root argument to twalk() is
one level of indirection less than the rootp arguments
to tsearch() and tdelete().

There are two nomenclatures used to refer to the order in which
tree nodes are visited. tsearch() uses preorder,
postorder and endorder to refer respectively to visiting a node before
any of its children, after its left child and before its right, and
after both its children. The alternate nomenclature uses preorder,
inorder and postorder to refer to the same visits, which could result
in some confusion over the meaning of postorder.

If the calling function alters the pointer to the root, the
results are unpredictable.

These functions safely allows concurrent access by multiple
threads to disjoint data, such as overlapping subtrees or tables.

Examples

Example 1 A sample program of using tsearch() function.

The following code reads in strings and stores structures containing
a pointer to each string and a count of its length. It then walks
the tree, printing out the stored strings and their lengths in alphabetical
order. The tree is destroyed at the end.